The present invention relates to the field of applying high-temperature superconduction in the power supply sector. It also relates to a device used in superconductor technology in accordance with the preamble of patent claim 1.
High-temperature superconductors are used, for example, in superconductive short-circuit current limiters for electrical distribution or transmission networks. A current limiter of this type makes use of the fact that a superconductor, at a suitably low operating temperature, only maintains its superconductivity while the current density of a current passing through it remains below a defined limit value which is referred to as the critical current density. If a short circuit occurs in the corresponding power supply network, the current in the current limiter rises to form a fault current which exceeds the critical value.
However, other superconductive components, such as transmission cables or transformers, also inherently have a critical current intensity (Ic). In rated operations, these components conduct a rated current IN, which is lower than the critical current Ic, without losses. However, transients which are generated by switching-network components such as filter circuits or large consumers in the supply network may cause this rated current to increase briefly to an overcurrent.
Devices based on high-temperature superconductors are normally immersed in a liquid cooling medium, which for the sake of simplicity is preferably liquid nitrogen (LN2), which is thermally insulated from the environment by a vessel known as a cryostat. As a result of a fault current or overcurrent in a superconductive component, the superconductor passes into the resistive state, i.e. the voltage which is applied at the corresponding section of the power supply network, at least for a short time, drops off completely (in the case of a short circuit) or partially at the superconductor. The Joule effect is generated, and under certain circumstances the superconductor is heated to well above its operating temperature, also heating the cooling medium which surrounds it. If this medium consists of liquid nitrogen, a large quantity of this liquid nitrogen is evaporated within a short time, so that an excess pressure is formed in the cryostat and pressure and shock waves propagate almost explosively. As a result, the superconductor device begins to vibrate and suffers irreparable mechanical damage, for example as a result of cracks being formed in the high-temperature superconductor ceramic or as a result of detachment of a conductor or support layer.
German laid-open specification DE-A 197 46 976 has shown a high-temperature superconductor arrangement for use in a current limiter. The arrangement comprises a superconductor layer and a perforated steel plate which is designed as an electrical bypass and, with the superconductor layer, forms a conductor assembly. In addition, the superconductor arrangement may be mechanically stabilized and electrically insulated by fiber-reinforced composite materials. In European patent EP-B 789 368, a vacuum-insulated superconductor device is connected, with good thermal conductivity, to a cryogenic cooler and is indirectly cooled by the latter.
It is an object of the present invention, in high-temperature superconductor applications in the field of power supply, to reduce the mechanical load on the superconductors and to increase their availability. This object is achieved by a device used in superconductor technology which has the features of patent claim 1 and by a method for operating this device which has the features of patent claim 10.
The core idea of the invention is to prevent evaporation of the cooling medium, with the associated occurrence of shock waves, in the event of abrupt heating of a superconductor arrangement and a cooling medium surrounding it. This is achieved by selecting as the cooling medium for the superconductor arrangement a fluid which does not undergo a phase transition even in the event of very slight heating. This cooling fluid is in thermal contact with a further cooling medium which forms a refrigeration reservoir with a nominal temperature (TN). Therefore, said cooling fluid is either a gas or a liquid with a boiling point (TB) which is higher than the nominal temperature TN, In normal or rated operation, the refrigeration reservoir determines an operating temperature of the superconductor arrangement which is, accordingly, only slightly above the nominal temperature TN. However, at the same time the refrigeration reservoir itself is only poorly thermally coupled to the superconductor and is therefore not affected, or is only affected with a delay, in the event of heating originating therefrom.
According to a first embodiment of the device according to the invention, the cooling fluid is the gaseous phase and the refrigeration reservoir the coexisting liquid phase of the same cooling medium. The nominal temperature TN corresponds to the boiling point (TB) of the cooling medium, which can be influenced by means of the pressure of its gas phase.
According to a second embodiment, the superconductor arrangement comprises a support and a conductor assembly which is applied to the support and is additionally thermally connected to the refrigeration reservoir via the support. The additional thermal conductivity provided by the support accelerates in particular the process of initial cooling from room temperature to operating temperature when the device is started up.
The problem of the explosive evaporation of cooling liquid is particularly acute in current limiters, since these are designed, in the event of a short circuit, to build up a considerable resistance and to dissipate large amounts of energy within a very short time. The risk to resistive current limiters comprising plate-like modules each having an electrically insulating support and conductor tracks applied on both sides increases as the surface extent of the modules rises, because, given a constant module thickness, the greater the surface extent of the modules, i.e. the surface area of the plates, the greater the vibration amplitudes become. Since, according to the invention, reduced mechanical loads occur, it is now also possible to use modules which have a surface extent of more than 0.05 m2.
In a further embodiment for current limiters having a plurality of modules, thermal compensation elements are provided between the modules for temperature homogenization purposes, which elements are preferably in contact with the refrigeration reservoir and themselves have a good thermal conductivity or are porous. The modules are preferably arranged in such a way that the spaces formed by them form a continuous passage for the cooling fluid. A pump may be used to assist the circulation of the cooling fluid.
The method according to the invention is characterized in that, in the case in which cooling fluid and refrigeration reservoir are two phases of the same cooling medium, during operation the superconductor arrangement is continuously wetted or showered with the liquid phase, i.e. a liquid cooling medium. Therefore, the gaseous cooling fluid forms continuously through evaporation of drops which strike the superconductor.
Further advantageous embodiments will emerge from the dependent patent claims.